Brake Booster Pressure Sensor Circuit Low


The PCM has detected a fault in the brake booster pressure sensor circuit.

Code Set Parameters

If the PCM fails to receive a voltage input signal from the brake booster pressure sensor that is proportionate to the decrease in vehicle speed required, a code will be stored and a malfunction indicator lamp will be illuminated. This problem normally occurs when the brakes are applied. Some vehicle models will require multiple ignition cycles with a failure of this nature for a check engine soon lamp to be illuminated but a code should be stored on the initial failure. If only the initial failure is exhibited, the code may be stored as a “pending code”.


Symptoms may include a hard brake pedal, increased effort required to depress the brake pedal, an illuminated service engine soon lamp, and a stored trouble code. In rare cases, the driver may experience no adverse drivability symptoms.

Common Causes

The most common cause of this code being stored is from vacuum leaks to the vacuum brake booster or a defective brake booster pressure sensor. Other causes may include shorted or open electrical circuitry and connectors, cracked or broken vacuum hoses to the brake booster, or a defective one way valve in the vacuum brake booster supply hose. Hydraulic brake boosters often fail due to power steering malfunctions, because the power steering pump also supplies hydraulic pressure to the booster. In this case, low power steering fluid is the most common culprit. A defective power steering pump may also cause low hydraulic brake booster pressure.

Common Misdiagnosis

While it is possible for the brake booster to fail, this condition is normally accompanied by a vacuum or hydraulic fluid leak. A vacuum supply problem, or sensor fault, are more likely causes but techs report that brake boosters are frequently replaced in error.


  • The brake booster pressure sensor is designed to provide the PCM with an input signal reflecting the amount of braking assist pressure available for stopping purposes
  • The PCM uses this input signal for the purpose of monitoring how much brake pressure is available to stop the vehicle at any given speed
  • Brake booster pressure sensors are generally of a silicon diaphragm design
  • A low voltage signal (typically 5 volts) and a ground signal are provided to the sensor
  • The higher the pressure becomes in the brake booster, the less resistance the sensor provides and the greater the voltage signal is to the PCM
  • If the brake booster pressure becomes insufficient, the resistance of the sensor becomes greater and signal voltage decreases to the PCM
  • In this scenario, the PCM recognizes the low voltage signal as low brake booster pressure and a trouble code is stored
  • Several tools will be instrumental in successfully diagnosing this code
  • A suitable OBD-II scanner (or code reader) and a digital volt/ohmmeter will be most helpful in performing a successful diagnosis
  • Begin with a visual inspection of all wiring and connectors
  • Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
  • Always retest the system after repairs are completed to ensure success. If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
  • This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
  • Continue by clearing the code and operating the vehicle to see if it returns
  • This will help to determine whether or not the malfunction is intermittent
  • After the codes are cleared, test drive the vehicle to see if the code returns
  • If the code fails to immediately return, you may have an intermittent condition
  • Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made
  • Proceed with your diagnosis with a visual inspection of the brake booster system
  • Make sure that there is no fluid leaking from the rear of the master cylinder into vacuum booster
  • Inspect all vacuum supply hoses used to supply the vacuum brake booster
  • If the system uses a hydraulic booster, visually inspect the power steering pump, lines, fluid condition, and fluid level
  • Also take a look at the pressure sensor, as well as the electrical wiring and connectors for signs of damage from corrosion, burning, or fluid contamination
  • Repair or replace faulty components as required. If the vacuum source checks out and no wiring or connectors are damaged, then connect the scanner (or code reader) and make a note of all stored codes and relevant freeze frame data
  • Test drive the vehicle and see if the code persists; if it does, then proceed with the diagnostic strategy. Disconnect the power brake booster sensor connector and check sensor resistance using the digital volt/ohmmeter
  • Compare your findings with manufacturer’s specifications and dispose of the sensor if actual readings do not coincide
  • If the sensor checks out, test the sensor connector for voltage and ground signals
  • If no voltage and ground signals are detected, disconnect the PCM connector and perform a continuity test on the entire circuit in order to isolate a possible malfunction and repair open or shorted wiring as required
  • If voltage and ground signals are present, connect the manual power steering pressure gauge or vacuum gauge
  • Use caution when testing hydraulic system pressure, as levels can reach the extremes
  • Consult your vehicle’s service manual and compare your findings, replace or repair power steering system or vacuum supply/engine components as needed.